Light Emitting Diode (LED) based lighting devices are being used for many applications. The relatively low power consumption and relatively long lifetime of LEDs make them a very useful alternative for conventional light sources like incandescent lamps or fluorescent tubes. Hence, not only new-designed lighting equipment often utilizes LEDs, but in many markets LED products are being used to replace other light sources like incandescent or halogen light sources, for instance. These so-called retrofit products have to be compatible with existing lighting/power supply systems.
In many lighting applications, dimmed light is needed. Conventional incandescent and halogen lamps have an effect that, when being dimmed, the color temperature of the light follows the Black Body Locus (BBL), referred to as the “BBL dimming effect”. The BBL dimming effect provides the visual effect of a warmer light generated by the lamp when dimmed than when being operated at maximum power, i.e. maximum light output level. This effect is used in ambiance light situations to provide a comfortable soft light, for instance in restaurants, lounges, living rooms etc.
The BBL dimming effect is also desired when using LED-based lighting devices. However, the LED light sources will provide light of substantially the same correlated color temperature regardless of the light output level. Therefore, to provide the BBL dimming effect, an amber LED light source may be provided in the LED-based lighting device, which amber LED provides light of a warmer color temperature. When the normal LED light sources are set to maximum light output level, the amber LED light source is switched off. This may provide light of a correlated color temperature of for example 2700K. When dimming the lighting device, the light output level of the normal LED light sources will be decreased, and at the same time, the light output level of the amber LED light source will be increased. At a minimum light output level of the device, the normal LED light sources will be switched off, and the amber LED light source will be at a maximum light output level. The light may then have a correlated color temperature of approximately 2200K.
It is a disadvantage of the known LED-based lighting devices that in order to achieve the BBL dimming effect a relatively complicated controller is required for controlling the light output level of the normal LED light sources and that of the amber LED light source, as it has to be determined what the required output levels should be in order to achieve the BBL dimming effect. It is a further disadvantage of the known LED-based lighting devices that the light output of the amber LED light source shows a relative large decrease at increasing temperature of the LED, which is in particular disadvantageous in applications where light of a high brightness, e.g. spot light applications, is required. It is another further disadvantage of the known LED-based lighting devices that the CRI (“Color Rendering Index”) of the generated light is relatively low at a color temperature of approximately 2200 K.
US2013/0093362A1) discloses a tunable light emitting device including a first set of LEDs, a second set of LEDs and a remote wavelength conversion component comprising two or more regions with different phosphors. The device further comprises a dimmer and a control circuit. The dimmer is configured to generate a continuous range of output powers to be used for the tunable light emitting device, and the control circuit is configured to translate the generated output power into an adjustable power arrangement for the LEDs.
US2010/0207521A1) discloses a light emitting apparatus including a plurality of LEDs on a substrate and a first phosphor layer covering the LEDs. On top of the phosphor layer a second phosphor material is provided in the form of dots. The second phosphor material allows performing fine color adjustments.